If you are interested in writing your Master's thesis with me, please take a look at my research and lecture material to get an understanding of which topics I would be most willing to supervise. A Master's thesis I supervise has to be empirical and apply causal estimation techniques or other approaches I teach and work with. I also encourage Bachelor students to consider conducting an empirical study for their theses (less ambitious than for Master's students, obviously). 

If you have a broad idea of the topic, please do some initial research on the literature and try to gain some understanding of what empirical approach and data sources might be relevant. Then, you will be in a good position to have an initial meeting about supervising. Please then email me briefly describing your idea and I will give you feedback on whether it is suitable/doable and how it might be tailored to become more feasible. If you are currently enrolled in one of my courses, you can, of course, always approach me before or after the lecture.